In the future, our sun will be transformed into a solid crystalline white dwarf. This is how it will happen

About fifty years ago, astronomers predicted what our ultimate fate would be. According to the theory, the sun will exhaust its hydrogen fuel billions of years from now and expand to become a red giant, followed by throwing out the outer layers and becoming a white dwarf. After a few billion years of cooling, the interior will crystallize and become solid.

Until recently, astronomers had little evidence to back up this theory. However, thanks to ESA's Gaia Observatory astronomers can now observe hundreds of thousands of white dwarf stars with immense precision – measuring their distance, brightness and color. This, in turn, has enabled them to study what the future holds for our Sun when it is no longer the hot, yellow star we know and love today.

The study describing these findings has recently appeared in the journal Nature under the title "Nuclear crystallization and build-up in the cooling sequence of the emerging white dwarfs." The study was led by Pier-Emmanuel Tremblay, an assistant professor at the University of Warwick, and included several researchers from Warwick's Astronomy and Astrophysics Group, the Université de Montreal and the University of North Carolina.

The artist's impression of star development, where stars ultimately become black holes, neutron stars or white dwarfs, depending on their classification. Image: ESA

With regard to star development, decades of observations in combination with theoretical models have enabled astronomers to determine what will happen to a star based on the classification. Bigger stars (like blue super giants) eventually go supernova and become neutron stars or black holes, smaller stars like our Sun will throw their outer layers to become planetary nebulae and finally finish their life cycle as a white dwarf.

These ultra-dense stars continue to emit radiation when they cool down, a process that lasts for billions of years. Eventually, their interiors will be sufficiently cold – about 10 million ° C – that the extreme pressure exerted on their cores causes the material to crystallize and become solid. It is estimated that this will be the fate of up to 97% of the stars in the Milky Way, while the rest will be neutron stars or black holes.

Because white dwarfs are among the oldest stars in the universe, they are incredibly useful for astronomers. Because their life cycle is predictable, they are used as "cosmic clocks" to estimate the age of groups of nearby stars with a high degree of accuracy. But deciding what happens to white dwarfs towards the end of their life cycle has been challenging.

Earlier, astronomers were limited in the number of white dwarfs they could study. Everything that changed with the deployment of Gaia a space observatory that has in recent years accurately measured positions, distances and movements of stars to create the most detailed 3D space catalog ever made.

ESA's Gaia mission is currently on a five-year mission to map the stars in the Milky Way. Gaia has found evidence of a galactic collision that occurred between 300 million and 900 million years ago. Credit: ESA / ATG medialab; background: ESO / S. Brunier.

"In the past, we had distance for just a few hundred white dwarfs and many of them were in clusters, where they all have The same age. With Gaia we now have the distance, the brightness and the color of hundreds of thousands of white dwarfs for a large selection in the Milk's outer disc, spanning a range of initial masses and all sorts of ages. "

Gaia data to analyze more than 15,000 stellar remaining candidates within 300 light years of the Earth. From this test, they could identify a surplus in the number of stars (aka. A pileup) that had specific colors and brightnesses that did not correspond to any single mass or age.

This accumulation, once compared to evolutionary models of stars, appeared to coincide with the stage of development where the stars lose heat in large quantities. This process slows down the natural cooling process and causes the dead stars to stop dimming, causing them to appear up to 2 billion years younger than they actually are. 0

"This is the first direct evidence that white dwarves crystallize or transition from floating to solid," Tremblay explained in a press release from Warwick. "It was predicted fifty years ago that we would observe a pile-up in the number of white dwarves at certain brightnesses and colors due to crystallization and only now has this been observed."

This pattern, where brightness is not related to age, was one of the most important predictions of crystallization of white dwarfs 50 years ago. Now that astronomers have direct evidence of this process at work, it is likely that it affects our understanding of what spokesmen white dwarves should be included in.

"White dwarves are traditionally used for age rating of star populations as clusters of stars, the outer disc and The halo in our Milky Way, says Tremblay. "We will now need to develop better crystallization models to get more accurate estimates of the age of these systems." For example, while all white dwarves will crystallize at some point in their development, The time it takes varies depending on the star.More massive white dwarfs cool down faster and reach the temperature at which crystallization takes place earlier (about a billion years). Less white dwarfs, which is what our Sun will be, may require as much as six billion years to make the same transition.

"This means that billions of white dwarfs in our galaxy already have quit the process and are essentially crystal balls in the sky, "Tremblay said. Meanwhile, our Sun may be expected to undergo this transition within about another ten billion years. At that time, our Sun will have gone from its facade of the red giant, become a white dwarf and began the crystallization process.

The life cycle of a sun-like star, from its birth on the left side of the frame to its development in a red giant on the right after billions of years. Credit: ESO / M. Kornmesser

This is just the latest revelation that comes from the Gaia mission, which over the past five years has cataloged celestial objects in the Milky Way and nearby galaxies. Before the assignment ends (expected to happen in 2022), two more data releases are planned, with the DR3 edition scheduled for 2021 and the final edition still to be determined.